1. Field of the Invention
The present invention relates to an electromagnetic wave radar device mounted on a car, which helps improve external appearance of a car without impairing radar function, and is fabricated using a decreased number of parts at a reduced cost.
2. Prior Art
An electromagnetic wave radar device has heretofore been used as an obstacle sensor being mounted on a car.
The device of this kind has been disclosed in, for example, xe2x80x9cAntenna for Microwaves/Millimeter Waves and Peripheral Technologyxe2x80x9d, Nov. 21, 1993, issued by Mimatsu Data System, pp. 474-477, xe2x80x9cSymposium on Utilizing Electromagnetic Waves, Preliminary Papers on Promoting the Utilization of Quasi-Microwaves/millimeter wavesxe2x80x9d, June, 1990, issued by, Foundation, Electromagnetic Wave System Development Center, p. 111), and xe2x80x9cHandbook of Antenna Engineeringxe2x80x9d, Oct. 30, 1980, edited by the Academy of Electronic Data Communication, issued by Ohm Co., pp. 359-361.
FIG. 17 is a perspective view showing the appearance of a conventional electromagnetic wave radar device mounted on a car disclosed in the above-mentioned literature xe2x80x9cAntenna for Microwaves/Millimeter Waves and Peripheral Technologyxe2x80x9d, p. 477.
In FIG. 17, an electromagnetic wave radar body 1 mounted on a car is provided on an upper part of a bumper 3 of a vehicle 2 and is disposed at a central part of a front grille 4.
The electromagnetic wave radar body 1 is directly disposed on a front part of the vehicle 2, or is disposed in an opening formed in the front grille 4, emits electromagnetic waves of a predetermined frequency as a transmission beam W1 toward the front of the vehicle 2, and receives the electromagnetic waves reflected by an obstacle (not shown) as a reception beam W2.
A transmission/reception antenna in the electromagnetic wave radar body 1 is located on the travelling side of the vehicle 2, and is covered by a known radome (not shown) made of a material which permits the electromagnetic waves to pass through in order to prevent the antenna from the external damaging factors such as rain, dust and dirt and to prevent its performance from being deteriorated.
According to the device disclosed in the above-mentioned literature xe2x80x9cHandbook of Antenna Engineeringxe2x80x9d, an opening (not shown) is formed in the bumper 3, and the electromagnetic wave radar body 1 is disposed in the opening.
According to the conventional electromagnetic wave radar devices mounted on the car as described above, an opening wider than an area corresponding to the transmission/reception antenna is formed in the bumper 3 or in the front grille 4 in order to mount the electromagnetic radar body 1 on the car 2.
However, forming an opening in the bumper 3 or in the grille 4 imposes a limitation on designing the appearance of the car 2.
Moreover, despite the electromagnetic wave radar body 1 is disposed at the back of the opening in the front grille 4 so will not to deteriorate the appearance of the car 2, the pattern of the transmission/reception electromagnetic waves is affected by the front grille 4 irrespective of the material (metal or dielectric) of the front grille 4, causing the object-detecting performance of the electromagnetic radar body 1 to be greatly deteriorated.
In order to accomplish both the electric performance of the electromagnetic wave radar body 1 and the designing of appearance of the car 2, therefore, limitation is imposed on a place for installing the electromagnetic wave radar device 1.
In either device disclosed in the above-mentioned literatures, furthermore, a separate radome is necessary for protecting the electromagnetic wave radar body 1.
In the conventional electromagnetic wave radar devices mounted on the vehicles as described above, the opening must be formed in the bumper 3 or in the grille 4 for installing the electromagnetic wave radar body 1, leaving a problem of impairing the appearance of the car 2.
When the electromagnetic wave radar body 1 is installed on a portion of the front grille 4, furthermore, the radar function is impaired being affected by the grille member.
Besides, a separate radome must be provided to protect the electromagnetic wave radar body 1 from the external environment, resulting in an increase in the number of parts and in an increase in the cost.
The present invention is directed to an electromagnetic wave radar device mounted on a car at a reduced cost, by using the bumper as a radome portion deteriorating neither the radar function nor the appearance of the car, and using a decreased number of parts by giving attention to the fact that the bumper of a car is made of a dielectric.
An electromagnetic wave radar device mounted on a car according to the present invention comprises an electromagnetic wave radar body mounted on a car and for transmitting and receiving electromagnetic waves of a predetermined frequency, and a radome portion made of a material which permits the electromagnetic waves to transmit through and covers the electromagnetic radar body, the electromagnetic wave radar body being disposed on the back surface side of the bumper of the car, and the radome portion being formed by a portion of the bumper of the car.
In the electromagnetic wave radar device mounted on a car according to the present invention, when a relative permittivity of the bumper is denoted by xcex5c, an angle of incidence of the electromagnetic waves relative to the bumper by xcex8i, a natural number by n, and a wavelength of the electromagnetic waves in free space by xcexo, then, a thickness tc of the radome portion in the bumper is set to a value that satisfies the following formula,   tc  =            n      ⁢              xe2x80x83            ·      λ        ⁢          xe2x80x83        ⁢          o      /                        {                      2            ⁢                                          (                                                      ϵ                    ⁢                                          xe2x80x83                                        ⁢                    c                                    -                                                            sin                      2                                        ⁢                                          xe2x80x83                                        ⁢                    θ                    ⁢                                          xe2x80x83                                        ⁢                    i                                                  )                                              }                .            
In the electromagnetic wave radar device mounted on a car according to the present invention, furthermore, the radome portion in the bumper has a dielectric lens portion for focusing the electromagnetic waves.
In the electromagnetic wave radar device mounted on a car according to the present invention, the dielectric lens portion is constituted by a thick portion that is formed in the bumper integrally together.
In the electromagnetic wave radar device mounted on a car according to the present invention, the dielectric lens portion is secured to a portion on the back surface of the bumper and is constituted integrally with the bumper.
In the electromagnetic wave radar device mounted on a car according to the present invention, the dielectric lens portion is inserted in the opening of the bumper and is constituted integrally with the bumper.
In the electromagnetic wave radar device mounted on a car according to the present invention, the radome portion in the bumper has a polarizer unit for transforming linear polarization and circular polarization relative to each other, and the polarizer unit has corrugated grooves.
In the electromagnetic wave radar device mounted on a car according to the present invention, furthermore, when a phase constant in a medium of the electromagnetic waves transmitting through the polarizer unit is denoted by xcex2m, a phase constant of the electromagnetic waves in free space by xcex2o and a natural number by n, then, a depth d of the corrugated grooves is set to a value that satisfies the following formula,
|xcex2mxe2x88x92xcex2o|d=(2nxe2x88x921)xcfx80/2.
In the electromagnetic wave radar device mounted on a car according to the present invention, the polarizer unit is formed in the bumper integrally together.
In the electromagnetic wave radar device mounted on a car according to the present invention, the polarizer unit is secured to a portion on the back surface of the bumper and is constituted integrally with the bumper.
In the electromagnetic wave radar device mounted on a car according to the present invention, the polarizer unit is inserted in the opening in the bumper and is constituted integrally with the bumper.
In the electromagnetic wave radar device mounted on a car according to the present invention, the radome portion in the bumper is coated with a coating material in a predetermined thickness, the coating material being composed of a material which permits the electromagnetic waves to transmit through.
In the electromagnetic wave radar device mounted on a car according to the present invention, furthermore, the coating material is applied onto at least either the front surface or the back surface of the radome portion of the bumper and, when a relative permittivity of the bumper is denoted by xcex5c, a relative permittivity es of the coating material is set to a value that satisfies the following formula,
xcex5c=xcex5s2.
and, when a natural number is denoted by n and a wavelength of the electromagnetic waves in free space by xcexo, a thickness ts of the coating material is set to a value that satisfies the following formula,
ts={(2nxe2x88x921)/4}xc2x7xcexo/{square root over (xcex5s)}.
In the electromagnetic wave radar device mounted on a car according to the present invention, furthermore, the radome portion in the bumper has a central portion which is directly related to transmitting and receiving the electromagnetic waves, and a peripheral portion surrounding the central portion, the peripheral portion being coated with an radiowave absorber on the side of the back surface thereof that faces the electromagnetic wave radar body.